]>
PCEP Procedures and Protocol Extensions for
Using PCE as a Central Controller (PCECC) of LSPsHuawei Technologies125 Nagog Technology ParkActonMA01719USAquintin.zhao@huawei.comHuawei TechnologiesHuawei Bld., No.156 Beiqing Rd.Beijing 100095Chinalizhenbin@huawei.comHuawei
TechnologiesDivyashree Techno Park, WhitefieldBangaloreKarnataka560066Indiadhruv.ietf@gmail.comHuawei TechnologiesDivyashree Techno Park, WhitefieldBangaloreKarnataka560066Indiasatishk@huawei.comJuniper Networks, IncUKadrian@olddog.co.ukCisco Systemschao.zhou@cisco.comRouting
PCE Working GroupThe Path Computation Element (PCE) is a core component of Software-
Defined Networking (SDN) systems. It can compute optimal paths for
traffic across a network and can also update the paths to reflect
changes in the network or traffic demands.PCE was developed to derive paths for MPLS Label Switched Paths
(LSPs), which are supplied to the head end of the LSP using the Path
Computation Element Communication Protocol (PCEP). But SDN has a broader
applicability than signaled (G)MPLS traffic-engineered
(TE) networks, and the PCE may be used to determine paths in a range
of use cases. PCEP has been proposed as a control protocol for
use in these environments to allow the PCE to be fully enabled as a
central controller.A PCE-based central controller (PCECC) can
simplify the processing of a distributed control plane by blending it
with elements of SDN and without necessarily completely replacing it.
Thus, the LSP can be
calculated/setup/initiated and the label forwarding entries can also be
downloaded through a centralized PCE server to each network devices
along the path while leveraging the existing PCE technologies as
much as possible.This document specifies the procedures and PCEP protocol extensions for
using the PCE as the central controller.
The Path Computation Element (PCE) was developed to offload
path computation function from routers in an MPLS traffic-engineered
network. Since then, the role and function of the PCE has grown to
cover a number of other uses (such as GMPLS ) and to allow
delegated control and PCE-initiated use of network
resources .According to , Software-Defined Networking (SDN) refers to a
separation between the control elements and the forwarding components
so that software running in a centralized system, called a
controller, can act to program the devices in the network to behave
in specific ways. A required element in an SDN architecture is a
component that plans how the network resources will be used and how
the devices will be programmed. It is possible to view this
component as performing specific computations to place traffic flows
within the network given knowledge of the availability of network
resources, how other forwarding devices are programmed, and the way
that other flows are routed. This is the function and purpose of a
PCE, and the way that a PCE integrates into a wider network control
system (including an SDN system) is presented in .In early PCE implementations, where the PCE was used to derive paths
for MPLS Label Switched Paths (LSPs), paths were requested by network
elements (known as Path Computation Clients (PCCs)), and the results
of the path computations were supplied to network elements using the
Path Computation Element Communication Protocol (PCEP) .
This protocol was later extended to allow a PCE to send unsolicited
requests to the network for LSP establishment . introduces the architecture for PCE as a central
controller as an extension of the architecture described in
and assumes the continued use of PCEP as the protocol used between
PCE and PCC. further examines the motivations and applicability
for PCEP as a Southbound Interface (SBI), and introduces the implications for the
protocol. describes the use cases for
the PCECC architecture.A PCE-based central controller (PCECC) can
simplify the processing of a distributed control plane by blending it
with elements of SDN and without necessarily completely replacing it.
Thus, the LSP can be
calculated/setup/initiated and the label forwarding entries can also be
downloaded through a centralized PCE server to each network devices
along the path while leveraging the existing PCE technologies as
much as possible.This draft specify the procedures and PCEP protocol extensions for
using the PCE as the central controller for static LSPs, where
LSPs can be provisioned as explicit label instructions at each
hop on the end-to-end path. Each router along the path must be
told what label-forwarding instructions to program and what resources
to reserve. The PCE-based controller keeps a view of the network and
determines the paths of the end-to-end LSPs, and the controller uses PCEP to
communicate with each router along the path of the end-to-end LSP. The extension for PCECC in Segment Routing (SR) is specified in a
separate draft .The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL
NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED",
"MAY", and "OPTIONAL" in this document are to be interpreted as
described in BCP 14 when, and only when, they
appear in all capitals, as shown here.Terminologies used in this document is same as described in the draft
and .In this mode LSPs are provisioned as explicit label instructions at each
hop on the end-to-end path. Each router along the path must be
told what label forwarding instructions to program and what resources
to reserve. The controller uses PCEP to communicate with each router
along the path of the end-to-end LSP.Note that the PCE-based controller will take responsibility for
managing some part of the MPLS label space for each of the routers
that it controls, and may taker wider responsibility for partitioning
the label space for each router and allocating different parts for
different uses. This is also described in section 3.1.2. of
. For the purpose
of this document, it is assumed that label range to be used by a PCE
is known and set on both PCEP peers. A future extension could add this capability to
advertise the range via possible PCEP extensions as well.
The rest of processing is similar
to the existing stateful PCE mechanism.Following key requirements associated PCECC should be considered when
designing the PCECC based solution:PCEP speaker supporting this draft MUST have the capability to
advertise its PCECC capability to its peers.PCEP speaker not supporting this draft MUST be able to reject
PCECC related extensions with a error reason code that indicates that this feature is not
supported.PCEP speaker MUST provide a means to identify PCECC based LSP in the
PCEP messages.PCEP procedures SHOULD provide a means to update (or cleanup) the label-
download entry to the PCC.PCEP procedures SHOULD provide a means to synchronize the labels between
PCE to PCC in PCEP messages.Active stateful PCE is described in . PCE
as a central controller (PCECC) reuses existing Active stateful PCE
mechanism as much as possible to control the LSP.This document defines the following new PCEP messages and extends the
existing messages to support PCECC:a PCEP message described in .
PCRpt message is used to send PCECC LSP Reports. It is also extended to report the set of Central Controller's Instructions (CCI) (label forwarding instructions in the context of this document) received
from the PCE. See for more details.a PCEP message described in .
PCInitiate message is used to setup PCE-Initiated LSP based on PCECC mechanism.
It is also extended for Central Controller's Instructions (CCI) (download or cleanup the Label forwarding instructions in the context of this document) on all nodes along the path.a PCEP message described in .
PCUpd message is used to send PCECC LSP Update.The new LSP functions defined in this document are mapped onto the messages as
shown in the following table.This document specify a new object CCI (see ) for the encoding of central controller's instructions. In the scope of this document this is limited to Label forwarding instructions. The CC-ID is the unique identifier for the central controller's instructions in PCEP. The PCEP messages are extended in this document to handle the PCECC operations.During PCEP Initialization Phase, PCEP Speakers (PCE or PCC)
advertise their support of PCECC extensions.This document defines a new Path Setup Type (PST) for PCECC, as follows:PST = TBD: Path is setup via PCECC mode.A PCEP speaker MUST indicate its support of the function described
in this document by sending a PATH-SETUP-TYPE-CAPABILITY TLV in the
OPEN object with this new PST included in the PST list.This document also defines the PCECC Capability sub-TLV . PCEP
speakers use this sub-TLV to exchange information about their PCECC
capability. If a PCEP speaker includes PST=TBD in the PST List of the
PATH-SETUP-TYPE-CAPABILITY TLV then it MUST also include the
PCECC Capability sub-TLV inside the PATH-SETUP-TYPE-CAPABILITY TLV. The presence of the PST and PCECC Capability sub-TLV in PCC's OPEN Object
indicates that the PCC is willing to function as a PCECC client.The presence of the PST and PCECC Capability sub-TLV in PCE's OPEN message
indicates that the PCE is interested in function as a PCECC server.The PCEP protocol extensions for PCECC MUST NOT be used if one or
both PCEP Speakers have not included the PST or the PCECC Capability sub-TLV in their
respective OPEN message. If the PCEP Speakers support the extensions
of this draft but did not advertise this capability then a PCErr message with
Error-Type=19(Invalid Operation) and Error-Value=TBD (Attempted
PCECC operations when PCECC capability was not
advertised) will be generated and the PCEP session will be terminated.A PCC or a PCE MUST include both PCECC-CAPABILITY sub-TLV and
STATEFUL-PCE-CAPABILITY TLV () (with I flag set ) in OPEN Object to support the extensions defined
in this document. If PCECC-CAPABILITY sub-TLV is advertised and STATEFUL-PCE-CAPABILITY TLV
is not advertised in OPEN Object, it SHOULD send a PCErr message with Error-Type=19
(Invalid Operation) and Error-value=TBD (stateful PCE capability was not advertised)
and terminate the session. The PCEP messages pertaining to PCECC MUST include PATH-SETUP-TYPE
TLV in the SRP object
to clearly identify the PCECC LSP is intended.In order to setup a LSP based on PCECC mechanism, a PCC MUST delegate the LSP by
sending a PCRpt message with PST set for PCECC (see ) and D (Delegate)
flag (see ) set in the LSP object.LSP-IDENTIFIER TLV MUST be included for PCECC LSP, the tuple uniquely identifies the LSP in the network. The LSP object is included in central controller's instructions (label download) to identify the PCECC LSP for this instruction. The PLSP-ID is the original identifier used by the ingress PCC, so the transit LSR could have multiple central controller instructions that have the same PLSP-ID. The PLSP-ID in combination with the source (in LSP-IDENTIFIER TLV) MUST be unique. The PLSP-ID is included for maintainability reasons. As per , the LSP object could include SPEAKER-ENTITY-ID TLV to identify the PCE that initiated these instructions. Also the CC-ID is unique on the PCEP session as described in .When a PCE receives PCRpt message with D flags and PST Type set, it calculates the path and assigns labels along the
path; and set up the path by sending PCInitiate message to each node
along the path of the LSP. The PCC generates a Path Computation State Report
(PCRpt) and include the central controller's instruction (CCI) and the identified LSP. The CC-ID is uniquely identify the central controller's instruction within PCEP. The PCC further responds with the PCRpt messages including the CCI and LSP objects.Once the central controller's instructions (label operations) are completed, the PCE SHOULD send the PCUpd message to the
Ingress PCC. The PCUpd message is as per SHOULD include the path information as calculated by the PCE.Note that the PCECC LSPs MUST be delegated to a PCE at all times.
LSP deletion operation for PCECC LSP
is same as defined in . If the PCE receives
PCRpt message for LSP deletion then it does Label cleanup operation as
described in for the corresponding LSP.The Basic PCECC LSP setup sequence is as shown below.
| PCECC LSP
|PCC +--------+ | |
|Egress | | | |
+--------+ | | |
| | | |
||
| | | |
| ||
| | | |
| | ||
| | | |
| | |
The PCECC LSP are considered to be 'up' by default (on receipt of PCUpd message from PCE).
The Ingress MAY further choose to deploy a data plane check
mechanism and report the status back to the PCE via PCRpt message.The new central controller's instructions (CCI) for the label operations in PCEP is done via the PCInitiate message, by
defining a new PCEP Objects for CCI operations. Local label range of
each PCC is assumed to be known at both the PCC and the PCE. In order to setup an LSP based on PCECC, the PCE sends a PCInitiate message
to each node along the path to download the Label instruction as described in .
The CCI object MUST be included, along with the LSP object in the PCInitiate message. The LSP-IDENTIFIER TLV MUST be included in LSP object. The SPEAKER-ENTITY-ID TLV
SHOULD be included in LSP object.If a node (PCC) receives a PCInitiate message which includes a Label to download as part of CCI, that is out
of the range set aside for the PCE, it MUST send a PCErr message with Error-type=TBD
(PCECC failure) and Error-value=TBD (Label out of range) and MUST include the
SRP object to specify the error is for the corresponding label update via PCInitiate message.
If a PCC receives a PCInitiate message but failed to download
the Label entry, it MUST send a PCErr message with Error-type=TBD
(PCECC failure) and Error-value=TBD (instruction failed) and MUST include the
SRP object to specify the error is for the corresponding label update via PCInitiate message.New PCEP object for central control instructions (CCI) is defined in .In order to delete an LSP based on PCECC, the PCE sends a central controller instructions via a PCInitiate
message to each node along the path of the LSP to cleanup the Label forwarding instruction.
If the PCC receives a PCInitiate message but does not recognize the
label in the CCI, the PCC MUST generate a PCErr message with Error-Type
19(Invalid operation) and Error-Value=TBD, "Unknown Label" and
MUST include the SRP object to specify the error is for the
corresponding label cleanup (via PCInitiate message).
The R flag in the SRP object defined in specifies
the deletion of Label Entry in the PCInitiate message.
| PCECC LSP
|PCC +--------+ | | remove
|Egress | | | |
+--------+ | | |
| | | |
||
| | | |
| ||
| | | |
| | ||
| | | |
]]>
As per , following the removal of the Label forwarding instruction, the PCC MUST send a PCRpt message.
The SRP object in the PCRpt MUST include the
SRP-ID-number from the PCInitiate message that triggered the removal.
The R flag in the SRP object MUST be set.The LSP Instantiation operation is same as defined in .In order to setup a PCE Initiated LSP based on the PCECC mechanism, a PCE
sends PCInitiate message with Path Setup Type set for PCECC
(see ) to the Ingress PCC.The Ingress PCC MUST also set D (Delegate) flag (see
) and C (Create) flag
(see ) in LSP object of
PCRpt message. The PCC responds with first PCRpt message with the status as "GOING-UP" and assigned PLSP-ID.Note that the label forwarding instructions from PCECC are send after the initial PCInitiate and PCRpt exchange. This is done so that the PLSP-ID and other LSP identifiers can be obtained from the ingress and can be included in the label forwarding instruction in the next PCInitiate message. The rest of the PCECC LSP setup operations are same as those described in .The LSP deletion operation for PCE Initiated PCECC LSP is same as defined
in . The PCE should further
perform Label entry cleanup operation as described in
for the corresponding LSP.The PCE Initiated PCECC LSP setup sequence is shown below -
| PCECC LSP
+--------+ | | (GOING-UP) |
| | | |
||
| | | |
| ||
| | | |
| | ||
| | | |
| | | |
| | | (UP) |
]]>
Once the label operations are completed, the PCE SHOULD send the PCUpd message to the
Ingress PCC. The PCUpd message is as per .In case of a modification of PCECC LSP with a new path, a PCE sends
a PCUpd message to the
Ingress PCC. But to follow the make-before-break procedures, the PCECC first update new instructions based on the updated LSP and then update to ingress to switch traffic, before cleaning up the old instructions. A new CC-ID is used to identify the updated instruction, the existing identifiers in the LSP object identify the existing LSP. Once new instructions are downloaded, the PCE further updates the new path at the ingress which triggers the traffic switch on the updated path. The Ingress PCC acknowledges with a PCRpt message, on receipt of PCRpt message, the PCE does cleanup operation for the old LSP as described in .The PCECC LSP Update sequence is shown below -
|
| | | |
| ||
| | | |
| | ||
| | | |
| | || Trigger
| | | (SRP=S) | Delete old
| | | | instruct
| | | |
||
| | | |
| ||
| | | |
| | ||
| | | |
]]>
The modified PCECC LSP are considered to be 'up' by default.
The Ingress MAY further choose to deploy a data plane check
mechanism and report the status back to the PCE via PCRpt message.As described in , a new PCE can gain control over the orphaned LSP. In case of PCECC LSP, the new PCE MUST also gain control over the central controllers instructions in the same way by sending a PCInitiate
message that includes the SRP, LSP and CCI objects and carries the
CC-ID and PLSP-ID identifying the instruction, it wants to take control of. Further, as described in , the State Timeout Interval timer ensures that a PCE crash does not
result in automatic and immediate disruption for the services using
PCE-initiated LSPs. Similarly the central controller instructions are not removed immediately
upon PCE failure. Instead, they are cleaned up on the expiration of
this timer. This allows for network cleanup without manual
intervention. The PCC MUST support removal of CCI as
one of the behaviors applied on expiration of the State Timeout
Interval timer.The purpose of Central Controllers Instructions synchronization (labels in the context of this document) is to make sure that the
PCE's view of CCI (Labels) matches with the PCC's Label allocation.
This synchronization is performed as part of the LSP state synchronization
as described in and
.As per LSP State Synchronization , a PCC reports the state of
its LSPs to the PCE using PCRpt messages and as per , PCE would
initiate any missing LSPs and/or remove any LSPs that are not wanted. The same PCEP messages and procedure is
also used for the Central Controllers Instructions synchronization. The PCRpt message includes the CCI and the LSP object to report the label forwarding instructions. The PCE would further
remove any unwanted instructions or initiate any missing instructions.As mentioned before, an Ingress PCC MAY choose to apply any OAM mechanism to check the status
of LSP in the Data plane and MAY further send its status in PCRpt message to the PCE. As defined in , a PCEP message consists of a common header
followed by a variable-length body made of a set of objects that can
be either mandatory or optional. An object is said to be mandatory
in a PCEP message when the object must be included for the message to
be considered valid. For each PCEP message type, a set of rules is
defined that specify the set of objects that the message can carry.
An implementation MUST form the PCEP messages using the object
ordering specified in this document.LSP-IDENTIFIERS TLV MUST be included in the LSP object for PCECC
LSP.The PCInitiate message can be used to download or remove the labels, the message has been extended as shown below -
::=
Where:
is defined in [RFC5440]
::=
[]
::=
(|
|
)
::= ::=
[]
Where:
and
are as per
[RFC8281].
The LSP and SRP object is defined in [RFC8231].
]]>When PCInitiate message is used for central controller's instructions (labels), the SRP, LSP and CCI objects MUST be present.
The SRP object is defined in and if the SRP object is missing, the receiving PCC MUST send
a PCErr message with Error-type=6 (Mandatory Object missing) and
Error-value=10 (SRP object missing). The LSP object is defined in and if the LSP object is missing, the receiving PCC MUST send
a PCErr message with Error-type=6 (Mandatory Object missing) and
Error-value=8 (LSP object missing). The CCI object is defined in and if the CCI object is
missing, the receiving PCC MUST send a PCErr message with Error-type=6
(Mandatory Object missing) and Error-value=TBD (CCI object missing).
More than one CCI object MAY be included in the PCInitiate message
for the transit LSR.To cleanup the SRP object must set the R (remove) bit.At max two instances of CCI object would be included in case of transit LSR to encode both in-coming and out-going label forwarding instructions. Other instances MUST be ignored.The PCRpt message can be used to report the labels that were allocated by the PCE, to be used during the state synchronization phase.
::=
Where:
::= []
::= (|
)
::= []
::= []
::=
[]
Where:
is as per [RFC8231] and the LSP and SRP object are
also defined in [RFC8231].
]]>When PCRpt message is used to report the central controller's instructions (labels), the LSP and CCI objects MUST be present.
The LSP object is defined in and if the LSP object is missing, the receiving PCE MUST send
a PCErr message with Error-type=6 (Mandatory Object missing) and
Error-value=8 (LSP object missing). The CCI object is defined in and if the CCI object is
missing, the receiving PCC MUST send a PCErr message with Error-type=6
(Mandatory Object missing) and Error-value=TBD (CCI object missing).
Two CCI object can be included in the PCRpt message
for the transit LSR.The PCEP objects defined in this document are compliant with the PCEP object
format defined in . This document defines a new optional TLVs for use in the OPEN Object.The PCECC-CAPABILITY sub-TLV is an optional TLV for use in the OPEN Object
for PCECC capability advertisement in PATH-SETUP-TYPE-CAPABILITY TLV. Advertisement of the PCECC capability
implies support of LSPs that are setup through PCECC as per PCEP extensions
defined in this document.Its format is shown in the following figure:The type of the TLV is TBD and it has a fixed length of 4 octets.The value comprises a single field - Flags (32 bits).No flags are assigned right now.Unassigned bits are considered reserved. They MUST be set to 0 on
transmission and MUST be ignored on receipt.The PATH-SETUP-TYPE TLV is defined in ;
this document defines a new PST value:
PST = TBD: Path is setup via PCECC mode.On a PCRpt/PCUpd/PCInitiate message, the PST=TBD in PATH-SETUP-TYPE TLV
in SRP object indicates that this LSP was setup via a PCECC
based mechanism.The Central Control Instructions (CCI) Object is used by the PCE to specify the forwarding instructions (Label information in the context of this document) to the PCC, and
MAY be carried within PCInitiate or PCRpt message for label download.CCI Object-Class is TBD.CCI Object-Type is 1 for the MPLS Label.The fields in the CCI object are as follows:
A PCEP-specific identifier for the CCI
information. A PCE creates an CC-ID for each instruction, the value is
unique within the scope of the PCE and is constant for the lifetime
of a PCEP session. The values 0 and 0xFFFFFFFF are reserved
and MUST NOT be used. is used to carry any additional information
pertaining to the CCI. Currently, the following flag bit is
defined: O bit(Out-label) : If the bit is set, it specifies the label is
the OUT label and it is mandatory to encode the next-hop
information (via IPV4-ADDRESS TLV or
IPV6-ADDRESS TLV or UNNUMBERED-IPV4-ID-ADDRESS TLV in
the CCI object). If the bit is not set, it specifies the label is
the IN label and it is optional to encode the local interface
information (via IPV4-ADDRESS TLV or
IPV6-ADDRESS TLV or UNNUMBERED-IPV4-ID-ADDRESS TLV in
the CCI object).The Label information.Set to zero while sending, ignored on receive.This document defines the following TLVs for the CCI object to
associate the next-hop information in case of an outgoing label and
local interface information in case of an incoming label.The address TLVs are as follows:
an IPv4 address.an IPv6 address.a pair of Node ID / Interface ID tuples.The security considerations described in and
apply to the extensions described in
this document. Additional considerations related to a malicious PCE are introduced.PCE has complete control over PCC to update the labels and can
cause the LSP's to behave inappropriate and cause cause major impact
to the network. As a general precaution, it is RECOMMENDED that
these PCEP extensions only be activated on authenticated and encrypted
sessions across PCEs and PCCs belonging to the same administrative
authority, using Transport Layer Security (TLS) ,
as per the recommendations and best current practices in . A PCE or PCC implementation SHOULD allow to configure to
enable/disable PCECC capability as a global configuration. describes the PCEP MIB, this MIB can be extended to get the
PCECC capability status.The PCEP YANG module could be extended
to enable/disable PCECC capability.Mechanisms defined in this document do not imply any new liveness
detection and monitoring requirements in addition to those already
listed in .Mechanisms defined in this document do not imply any new operation
verification requirements in addition to those already listed in
and .PCEP extensions defined in this document do not put new requirements
on other protocols.PCEP extensions defined in this document do not put new requirements
on network operations.IANA is requested to confirm the early allocation of the following
TLV Type Indicator values within the "PCEP TLV Type Indicators" sub-
registry of the PCEP Numbers registry, and to update the reference in
the registry to point to this document, when it is an RFC:ValueMeaningReferenceTBDPCECC-CAPABILITYThis documentTBDIPV4-ADDRESS TLVThis documentTBDIPV6-ADDRESS TLVThis documentTBDUNNUMBERED-IPV4-ID-ADDRESS TLVThis documentIANA is requested to allocate new PST Field in PATH-
SETUP-TYPE TLV. The allocation policy for this new registry should
be by IETF Consensus. The new registry should contain the following value:ValueDescriptionReferenceTBDTraffic engineering path isThis documentsetup using PCECC modeIANA is requested to allocate new registry for CCI PCEP object.Object-Class ValueNameReferenceTBDCCI Object-TypeThis document1MPLS Label IANA is requested to create a registry to manage the Flag field
of the CCI object.One bit to be defined for the CCI Object flag field in this document:Codespace of the Flag field (CCI Object)BitDescriptionReference7Specifies labelThis document is out labelIANA is requested to allocate new error types and error values within
the "PCEP-ERROR Object Error Types and Values" sub-registry of the
PCEP Numbers registry for the following errors:
MeaningInvalid operation.
Attempted PCECC operations when PCECC capability was not advertisedStateful PCE capability was not advertisedUnknown LabelMandatory Object missing.
CCI object missingPCECC failure.
Label out of range.Instruction failed.We would like to thank Robert Tao, Changjing Yan, Tieying Huang and Avantika for
their useful comments and suggestions.